Traditionally, digitally controlled color inkjet printing is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which drops of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million shades or color combinations.
The first technology, commonly referred to as “drop on demand” inkjet printing, selectively provides ink drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink drops, as required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle helping to keep the nozzle clean.
Conventional drop on demand inkjet printers utilize a heat actuator or a piezoelectric actuator to produce the ink drop at orifices of a printhead. With heat actuators, a heater, placed at a convenient location, heats the ink to cause a localized quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink drop to be expelled. With piezoelectric actuators, a mechanical force causes an ink drop to be expelled.
The second technology, commonly referred to as “continuous stream” or simply “continuous” inkjet printing, uses a pressurized ink source that produces a continuous stream of ink drops. Traditionally, the ink drops are selectively electrically charged. Deflection electrodes direct those drops that have been charged along a flight path different from the flight path of the drops that have not been charged. Either the deflected or the non-deflected drops can be used to print on receiver media while the other drops go to an ink capturing mechanism (catcher, interceptor, gutter, etc.) to be recycled or disposed. U.S. Pat. No. 1,941,001 (Hansell) and U.S. Pat. No. 3,373,437 (Sweet et al.) each disclose an array of continuous inkjet nozzles wherein ink drops to be printed are selectively charged and deflected towards the recording medium.
In another form of continuous inkjet printing, such as is described in commonly-assigned U.S. Pat. No. 6,491,362 (Jeanmaire), included herein by reference, stimulation devices are associated with various nozzles of the printhead. These stimulation devices perturb the liquid streams emanating from the associated nozzle or nozzles in response to drop formation waveforms supplied to the stimulation devices by control means. The perturbations initiate the separation of a drop from the liquid stream. Different waveforms can be employed to create drops of a plurality of drop volumes. A controlled sequence of waveforms supplied to the stimulation device yields a sequence of drops, whose drop volumes are controlled by the waveforms used. A drop deflection means applies a force to the drops to cause the drop trajectories to separate based on the size of the drops. Some of the drop trajectories are allowed to strike the print media while others are intercepted by a catcher or gutter.
In this form of continuous inkjet printing, typically a printhead includes a large number of nozzles formed on a nozzle plate, with each nozzle having an associated stimulation device that is also formed on the nozzle plate. Since each stimulation device is typically activated by an independently controlled sequence of waveforms, a large number of electrical connections must be made between the stimulation devices on the nozzle plate and the drop formation mechanism control circuit that provides the sequences of waveforms. Typically the drop forming mechanism control circuitry is also formed on the nozzle plate to reduce the number of electrical connections that must be made to the nozzle plate. The drop forming mechanism control circuitry formed on the nozzle plate is typically formed using a CMOS process. The drop forming mechanism control circuit receives a set of waveforms and waveform selection control information from an image synchronization controller, which is typically located on a circuit board.
In this printing system, typically two volumes of drops are used, a small drop having a small drop volume and a large drop whose volume is approximately N times the small drop volume, where N is an integer. Small drops are formed by small drop waveforms having a period, called the small drop period. Large drops are formed by large drop waveforms having a large drop period equal to N times the small drop period. The small drop frequency, the inverse of the small drop period, serves as the base or fundamental frequency for drop formation. The base, or fundamental, drop creation rate or frequency is typically fixed, or at least cannot be varied widely. In some cases the base drop creation frequency is defined by a printing system clock or by a natural characteristic of the drop generator such as its resonant frequency.
As described in commonly assigned U.S. Pat. No. 7,828,420 (Fagerquist et al), the large drop waveform can include a number of activation pulses within the large drop period to improve the formation or coalescence time of the large drop, uniformity of drop velocity, and the drop-to-drop spacing. As discussed therein, the large drop waveform can influence the uniformity of drop velocity and drop-to-drop spacing for small drops formed after the large drop formed by the large drop waveform. While the large drop waveform can be designed to improve the drop velocity uniformity of subsequent small drops, it is useful to provide more than one small drop waveform: one small drop waveform for use when the preceding drop is a large drop and another small drop waveform for use when the preceding drop is a small drop. Similarly, it is desirable to provide more than one large drop waveform: one large drop waveform for use when the preceding drop is a large drop and another large drop waveform for use when the preceding drop is a small drop. As the small drop period serves as the basic time period for drop formation, it is useful to define the large drop waveforms as defined sequences of large drop sub-waveforms, where each large drop sub-waveform has a period equal to the small drop period.
As the base drop frequency is fixed, or at least cannot be varied widely, and since there are a plurality of small drop waveforms and large drop sub-waveforms, the traditional method of controlling the sequence of drops formed by each nozzle in the printhead has involved the image synchronization controller providing all of the small drop waveforms and large drop sub-waveforms along with waveform selection control signals to the drop forming mechanism control circuit during each base drop period. Providing all of the waveforms and waveform selection control signals from the image synchronization controller to the drop forming mechanism control circuit during each base drop period requires many interconnects between the image synchronization controller and the drop forming mechanism control circuit. For example, in one implementation, there are eight unique waveforms for a 512-nozzle segment of the nozzle plate. The control circuitry associated with each nozzle requires a 3-bit waveform selection control signal to select one of the eight waveforms. This results in a total of 1536 select bits to be sent to the nozzle plate segment during each base drop period. The printhead operates with a base drop frequency of 480 kHz, resulting in a required bandwidth of approximately 750 megabits/second for the select signals. To keep the data rate low enough for the CMOS process used to fabricate the nozzle plate, the interconnect between the image synchronization controller and the nozzle plate segment that carries the waveform selection signals must be at least 8 bits wide. When combined with clock, latch, and enable signals necessary to operate the nozzle plate segment, this results in a total of 19 interconnects to control the nozzle plate segment. It is desirable to minimize the number of interconnects to the nozzle plate to reduce manufacturing costs and improve reliability.
It is also desirable to minimize the drop forming mechanism control circuitry on the nozzle plate to improve manufacturing yield and increase the number of nozzle plates that can be produced from one silicon wafer, thereby reducing the manufacturing cost.